Resonant actuator for optical scanning

ABSTRACT

An optical scanner or actuator comprising an armature which includes a movable, closed electrically conductive loop, the armature being mounted to rotate about a support axis, current inducing means associated with the electrically conductive loop comprising a stationary flux-conductor extending through and in current-inducing relationship with the loop, and drive means adapted to induce a magnetic current-inducing flux along the flux-conductor in accordance with desired movement of the armature, permanent magnet biasing means comprising a permanent magnet adapted to produce a magnetic field with which current in the electrically conductive loop interacts to cause movement of the armature, one pole of the magnet being permeably connected to one portion of the flux-conductor at a region remote from the electrically conductive loop, and the other pole of the magnet being permeably connected with the flux-conductor via a permeable gap in which a segment of the electrically conductive loop is arranged to move, whereby bias magnetic flux and current-inducing flux share in part the same permeable path along the flux-conductor, and a circuit for controlling the drive means to produce a varying flux in the flux-conductor to cause the armature to rotate back and forth about the support axis. In the form of a resonant scanner, an optical element is attached to the armature in the vicinity of the support axis and arranged to receive a beam to be directed by the optical element, and the control circuit produces signals to induce a varying current in the closed movable electrically-conductive loop to produce resonant scanning motion of the optical element.

BACKGROUND OF THE INVENTION

The invention relates to beam scanners and the like.

In scanners used for rapidly scanning a beam back and forth along aline, the optical scanning element oscillates about an axis with rotarymotion. In order to minimize the driving energy, it is often desiredthat the scanner be highly resonant, i.e. characterized by a spring-masssystem having a high (Q) factor. Certain resonant scanners are severelylimited in rotational speed or in the maximum angular excursion that canbe obtained. Other scanners are subject to internally inducedperturbations that cause detrimental wobbling of the optical element.

In cases where it is desired to employ externally generated drivesignals to control a scanner or other rotating oscillator, difficultyand expense are involved in seeking to match the drive signals to theoscillating mechanical system.

The invention is directed to overcoming these difficulties and achievingimproved scanning performance and simple and low cost actuatorconstructions.

SUMMARY OF THE INVENTION

In general, the invention features an optical scanner or actuatorcomprising an armature which includes a movable, closed electricallyconductive loop, the armature being mounted to rotate about a supportaxis, current inducing means associated with the electrically conductiveloop comprising a stationary flux-conductor extending through and incurrent-inducing relationship with the loop, and drive means adapted toinduce a magnetic current-inducing flux along the flux-conductor inaccordance with desired movement of the armature, permanent magnetbiasing means comprising a permanent magnet adapted to produce amagnetic field with which current in the electrically conductive loopinteracts to cause movement of the armature, one pole of the magnetbeing permeably connected to one portion of the flux-conductor at aregion remote from the electrically conductive loop, and the other poleof the magnet being permeably connected with the flux-conductor via apermeable gap in which a segment of the electrically conductive loop isarranged to move, whereby bias magnetic flux and current-inducing fluxshare in part the same permeable path along the flux-conductor, and acircuit for controlling the drive means to produce a varying flux in theflux-conductor to cause the armature to rotate back and forth about thesupport axis. In the form of a resonant scanner, an optical element isattached to the armature in the vininity of the support axis andarranged to receive a beam to be directed by the optical element, andthe control circuit produces signals to induce a varying current in theclosed movable electrically-conductive loop to produce resonant scanningmotion of the optical element.

In preferred embodiments, there is an armature-supporting structurehaving a flexural support constructed and arranged to impose arestorative force tending to return the armature from a deflectedposition to its neutral position, the armature and thearmature-supporting structure having a resonant frequency of rotationalmotion; the flexural support is a torsion bar assembly; the torsion barassembly has a pair of coaxial, axially spaced-apart torsion bars withthe armature held between the respective ends of the bars, the torsionbars defining the support axis; the armature is balanced and thearmature-supporting structure is arranged to prevent external forcesimposed in any direction on the armature-supporting structure fromcausing rotation of the armature about a predetermined axis normal tothe support axis; the motion of the armature is characterized by aquality factor for resonating system (Q) of at least 200, preferably inmany cases 1,000 or more; the circuit has a sensor responsive torotation of the armature, and feedback circuitry responsive to thesensor for providing control signals to the drive means; the sensor hasa pickoff coil wound around the flux-conductor for providing a signalindicative of the flux induced in the flux-conductor by rotation of thearmature; the feedback circuitry has an adjustable gain means forcontrolling the amplitude of rotational excursion of the armature; in anexternally driven system the permanent magnet is selected to providesubstantial magnetic damping that facilitates the matching of thefrequency of the driving signals to the natural oscillating frequency ofthe mechanical scanning system; the drive means has a coil wound aroundthe flux-conductor; and the flux-conductor and the permanent magnetstator comprise respective opposed, concentric, arcuate surfaces ofdifferent radii centered on the support axis, the surfaces defining anarcuate gap, the conductive loop having a segment disposed within thegap in a direction parallel to the support axis and normal to flux linesproduced by the permanent magnet and passing through the gap between thearcuate surfaces, whereby the force on the loop caused by theinteraction of the permanent magnetic field and the induced current inthe loop is uniform regardless of the angular position of the segment ofthe loop within the gap.

The actuator enables efficient, resonant, wide-excursion motion to beachieved in a compact, inexpensive unit and without susceptability toundesired wobbling of the optical element; the optical element can beattached close to the support axis to minimize rotational inertia; thesharing of flux paths by the drive means and the permanent magnetenables the pickoff coil to accurately determine the velocity of thearmature; the property of the permanent magnet, that it is not permeableto externally generated magnetic fields, prevents short circuiting ofthe flux induced by the drive coil; the rotational motion of thearmature need not be damped by bearings, brushes or leads connected tothe movable loop; the degree of damping of the system is selectable e.g.by selection of the strength of the permanent magnet; and the amplitudeof resonance is adjustable.

Other advantages and features will be apparent from the followingdescription of the preferred embodiment, and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

We first briefly describe the drawings.

Drawings

FIG. 1 is an isometric view, partially broken away, of a resonantscanner.

FIG. 2 is a cross sectional view (at 2-2 in FIG. 1) of the resonantscanner.

FIG. 3 is a block diagram of control circuitry.

FIG. 4 is a partially isometric view (not to scale) of a laser scanningsystem.

FIG. 5 is a diagrammatic view of a scanner, according to the invention,driven by an external source.

STRUCTURE

Referring to FIG. 1, scanner 10 has an armature 11 comprising arectangular closed, electrically conductive coil or loop 12 (L_(c) =0.25inches by W_(c) =0.200 inches) of 0.020 inch by 0.050 inch cross-section6061 T4 aluminum wire. Loop 12 is soldered to a torsion bar assemblycomprising a pair of 0.026 inch diameter, 1-inch long carbon springsteel torsion bars 14, each soldered on one end to a mirror/loop support16. Support 16 is a 0.124 inch diameter invar (0.003" copper-plated)piece, having flats cut on opposite sides, one flat providing a mountingsurface for the loop 12. The second flat provides a mounting surface fora 0.400 inch round, 1 millimeter thick silvered quartz flat mirror 18.

The opposite ends of the torsion bars 14 are each soldered to a 0.124inch diameter cylindrical bead 20. The pair of beads 20 are heldrespectively by clamps 22. One clamp 22 is screw mounted to the end ofan L-shaped aluminum support 24; the other clamp 22 is screw mounted toone end of a 0.020" thick spring steel blade 26, the other end of whichis secured to support 24, blade 26 being pre-stressed so that thetorsion bars 14 are placed under tension.

The torsion bar assembly, conductive loop and mirror form a mass-springsystem which can rotate about an axis y (colinear with bars 14) withvery low drag. The mass/spring system has a neutral position at whichmirror 18 lies in a plane defined by axis y and an axis z, axis x beingnormal to the surface of mirror 18 in this position. When mirror 18 isin any angular position other than the neutral position, torsion bars 14tend to return it to the neutral position with a force proportional tothe angular displacement from the neutral position.

Also mounted on stationary support 24 is 0.625 inch square permeablering 30 of 0.75 inch by 0.150 inch cross-section nickel iron (Carpenter49 available from Carpenter Steel Corporation) arranged to extendthrough the rotatable loop 12.

Referring to FIG. 2, in the region where it extends through conductiveloop 12, permeable ring 30 has a convexly rounded surface 32 with a0.215 inch radius centered on the y axis. Opposite surface 32 a polepiece 34 of nickel iron (Carpenter 49) has a corrresponding concavearcuate surface 36 of radius 0.285 inches centered on axis y, surfaces32 and 36 thereby defining a 0.070" gap g having an arcuate extent φ ofabout 60°, (providing a useful range of rotation up to about 35°). Theother end of pole piece 34 is permeably connected to the north pole ofsamarium cobalt permanent magnet 38 (having an energy of 20 milliongauss-oersteds). The south pole of magnet 38 is in turn permeablyconnected to permeable ring 30 at a region remote from the gap g.

Thus permanent magnet 38 establishes a flux across gap g and in eitherdirection, half-way around the ring 30 to the other pole of thepermanent magnet, with all of the flux lines across gap g beingsubstantially radial to axis of rotation y. Segment 40 of loop 12 (i.e.,the side opposite mirror 18) extends parallel with axis y and lieswithin gap g. It rotates through an arc within gap g as mirror 18rotates on axis y.

On one side of ring 30, a drive coil 42 having 250 turns of number 32copper wire (and a resistance of approximately 2.5 ohms) is wound and onthe opposite side of ring 30 is wound a pickoff coil 44 with the samespecifications.

Optical beam 50 is reflected from mirror 18 so that rotation of mirror18 causes reflected beam 52 to track a line. The torsion bar assembly isso designed and the mass of the mirror 18 and armature 11 are sobalanced about the various axes x, y and z that linear or rotationaldisturbances to scanner 10 do not shift the angle of the path of beam 52(with respect to axis x) regardless of the direction or axis of thedisturbance, except for certain correctable shifts caused by rotationaldisturbances about the y axis and rotational disturbances about the zaxis (which can be isolated by use of an appropriate mounting device,not shown, between support 24 and the chassis on which support 24 ismounted). Balancing armature 11 and mirror 18 with respect to axis yassures that linear disturbances in the x axis direction cause onlylinear displacement of mirror 18 along the z axis and a rotationaldisturbance about the x axis, of course, will not disturb the angle ofreflection of beam 52. Rotational disturbances about the y axis arecorrectable by the feedback loop (described below) which controlsrotation about the y axis. Rotation about the z axis is reduced to about1 arc-second by proper mounting of scanner 10 (e.g., on an H-shaped softsupport as in FIG. 4).

Referring to FIG. 3, scanner 10 has a lead 62 from pickoff coil 44 whichserves as an input to a feedback circuit for establishing resonantoscillations of and controlling the amplitude of the oscillations ofmirror 18. Band pass filter 64 uses the pickoff coil signal as its inputand provides a filtered velocity value to scaling amplifier 66 whichadjusts the signal to be in phase with the motion of scanner 10. Theoutput of phase shifter 68 is sent both to multiplier 70 and peakdetector 72. The signal peak value is sent to comparator 74 forcomparison with a set value from potentiometer 76 and the comparatoroutput is used to drive a proportional-integral-derivative controller 73which delivers its output to multiplier 70 for multiplication with thesignal from phase shifter 68. The output of multiplier 70 is fed throughamplifier 78 to drive coil lead 80 of scanner 10.

Referring to FIG. 4, in a so-called intelligent photocopying machine,scanner 10 line scans reflected laser beam 52 acrosslaser-beam-sensitive drum 82. Solid-state laser beam source 84, drumdrive 86 and scanner 10 are all connected to controller 88. In scanner10, ring 30, magnet 38 and pole piece 34 are all supported in moldedplastic housing 90 which is bolted to support 24. Housing 90 provides areceptacle for connecting drive coil and pickoff coil leads throughconventional cable connector 92 to controller 88. Support 24 is held onchassis 94 by H-shaped soft support 96, which absorbs rotationalperturbations to the chassis, preventing wobbling of mirror 18.

OPERATION

By applying a changing current to drive coil 42, flux is induced influx-conductor ring 30 in a direction which corresponds to the directionof change of the current. This flux induced in ring 30 in turn induces acurrent in the loop 12 of the armature whose magnitude and directiondepends on the magnitude and direction of the flux induced in ring 30.The reaction of the current in the loop of armature 12 against the fieldlines across gap g (set up by pole piece 34) impose a torque tending torotate armature 11 (and hence mirror 18) about axis y. The workingtorque is free of radial components so there is no tendency for mirror18 to move in any other direction or about any other axis than axis y.

The flux H induced in ring 30 is related to the current I in the drivecoil in accordance with Ampere's law, HL=NI, where L is the length ofthe ring and N is the number of turns in the drive coil. According toFaraday's law, the potential e on the armature, generated by flux H, is##EQU1## where A is the cross-sectional area and u is the permeabilityof ring 30. Assuming an alternating drive current of the form I=I_(o)sin wt (where w is the resonant frequency of the armature/supportsystem), the induced voltage is also alternating and of the form:##EQU2## and the resulting alternating current I_(a) in the armature(assuming it has a resistance R) is: ##EQU3## The resulting torque T onsegment 40 is:

    T=BlI.sub.a r                                              (4)

where l is the length of the portion of the armature loop in gap g, r isthe radius from the y axis to segment 40 of the armature, and B is thefield of magnet 38. Thus ##EQU4## that is, there is an oscillatingtorque imposed on the armature.

The torque will tend to cause armature 11 to rotate with angularvelocity (dθ/dt) which will induce a potential (E) in the armature loopdefined by ##EQU5## producing a current (I_(D)) in the armature loopwhich is and a corresponding damping torque (T_(D)) of opposite signfrom T,

    T.sub.D =BlI.sub.D r                                       (8)

or ##EQU6## Assuming that the motion of the armature is that of asecond-order system with inertia J and spring constant k with mechanicaldamping C, then the equation of motion of the armature when subjected tothe above-described torques will be ##EQU7## where the resonantfrequency of motion of the system ##EQU8## is (by operation of thefeedback loop) also the frequency of the driving coil current, so that##EQU9## which can be simplified to ##EQU10## and the solution is##EQU11## Where the damping ratio δ is defined as δ=(C/2w) the solutionsimplifies to ##EQU12## Thus the armature motion is resonant with thedamping ratio having two terms, the first based on the magnetic dampingof torque T_(D), the other based on the mechanical damping of thetorsion bar/armature system. It is desirable to have the magnetic andmechanical damping terms roughly matched and, for resonant operation, tohave the smallest possible values. The magnetic damping can be fixed ata desired level by appropriate selection of the strength of thepermanent magnet field, the length of the armature loop segment in thegap (g), the operation radius (r), and the resistance of the armatureloop (R). Note that the magnetic damping operates only with respect torotational motion about the y axis.

The mechanical resonance can be adjusted by bending small tabs onsupport 16 toward or away from the center of rotation thereby adjustingthe system's inertia.

The potential e_(p) at the pickoff coil 44 is a function of flux inducedin ring 30 from two sources, the drive coil current and the armaturemotion. The fluxes from the two sources share in part the same permeablepath along the flux-conductor.

With pickoff coil 44 and drive coil 42 having the same number of turns(N), ##EQU13## The angular velocity of the armature (dθ/dt) based on theabove solution for θ is ##EQU14## In this equation the velocity term##EQU15## (related to the angular velocity of the armature) and thetransformer term (i.e., l) (related to the drive coil current) are ofthe same phase and similar magnitude. Thus, by subtracting from e_(p) avoltage of proper amplitude and phase derived from drive coil 42, theresulting voltage will be proportional to the armature angular velocityand can be used in feedback control circuitry to maintain theoscillating motion of the armature at the resonant frequency and at acontrollable amplitude level.

Referring to FIG. 3, output line 62 from scanner 10 provides a voltageproportional to the angular velocity of armature 11 (i.e., equal topickoff coil 44 voltage e_(p) offset by the drive coil 42 voltage) whichis filtered in adjustable bandpass filter 64 which is tuned to theresonant frequency of scanner 10 to reduce distortion in the velocitysignal. The filtered signal is then scaled to a proper amplitude for usein the automatic gain control circuitry 71 to which it is fed. Incircuitry 71, peak detector 72 delivers a DC voltage equal to thepositive peak amplitude of the filtered and scaled velocity signal. ThatDC voltage is then fed to comparator 74 where it is compared with avariable reference voltage from potentiometer 76 (indicative of thedesired amplitude of resonance). The outputs of comparator 74 anddetector 72 are both bed to a conventionalproportional-integral-derivative control block 73 which has a transferfunction equal to ##EQU16## The integral term k₃ /S ensures that thesteady-state error of the scanner amplitude with respect to the variablereference signal will be zero. The output of block 73 controls the gainof multiplier 70.

Multiplier 70 receives its other input from the output of scalingamplifier 66 after it has been phase corrected in phase shifter 68 (toensure a 360° closed-loop phase shift) and passed through adjustableattenuator 69 (which assures that the input to the multiplier is withinthe multiplier's linear operating range). The output of multiplier 70provides the input to amplifier 78 which delivers a drive current todrive coil 42.

Scanner 10 has the following approximate specifications:

J=0.04 gram-cm² (mirror plus armature)

r=0.635 cm

l=0.312 cm

N=250 turns

I_(o) =0.2 amps

B=0.5 weber/m₂

L=6.35 cm

A=73×10⁻³ cm²

w=1 kHz=2π10³ radians/sec

R=0.868×10⁻³ ohms

u=5000×4π10⁻⁷

k=1.61×10³ gram-cm/radian

C=25×10⁻⁹ (mks system)

d=27.5×10⁻³

Q=182

θ=13.7 degrees (estimated) peak excursion

power consumption=1/4 watt

Referring again to FIG. 4, controller 88 can modulate the intensity oflaser source 84 in accordance with a desired changing image brightnessalong a succession of lines on a page to be generated. Controller 88coordinates the frequency and amplitude of the resonance of scanner 10and the advancement of drum 82 by drum drive 96 with the modulation ofthe intensity of source 84 so that the image scanned over the surface ofdrum 82 corresponds with the page to be generated. Scanning can proceedat 300 lines per inch at a speed of 1000 lines per second over an81/2"×11" surface with the scanner mirror rotating through 30°peak-to-peak.

The system can thus achieve with simple construction, a wide angleexcursion with a small motor and low power, with great accuracy and witha long expected life.

In another embodiment, FIG. 5, an external source 100 is employed toproduce the drive signals. Such an arrangement may be accomplished bydisconnecting the connection in FIG. 3 between altenator 69 andmultiplier 70 to break the feedback loop and feeding the drive sourcesignal to the multiplier. In this case the permanent magnet of thescanner is chosen to provide a considerably stronger field B, with theresult that the damping torque (see equations 8 and 9) is made higher asa result of magnetic damping. This lowers the Q of the spring-masssystem and by this, increases the range of permissible mismatch betweenthe frequency of the external drive signal and the natural resonantfrequency of the scanner, thus making it practical to employ arelatively low-cost external drive source and scanner.

OTHER EMBODIMENTS

Other embodiments are within the following claims.

I claim:
 1. A resonant optical scanner device comprisingan armaturewhich includes a movable, closed electrically conductive loop, saidarmature being mounted to rotate about a support axis, current inducingmeans associated with said electrically conductive loop comprisingastationary flux-conductor extending through and in current-inducingrelationship with said loop, and drive means adapted to induce amagnetic current-inducing flux along said flux conductor in accordancewith desired movement of said armature, permanent magnet biasing meanscomprisinga permanent magnet adapted to produce a magnetic field withwhich current in said electrically conductive loop interacts to causemovement of said armature,one pole of said magnet being permeablyconnected to one portion of said flux-conductor at a region remote fromsaid electrically conductive loop, and the other pole of said magnetbeing permeably connected with said flux-conductor via a permeable gapin which a segment of said electrically conductive loop is arranged tomove, whereby bias magnetic flux and current-inducing flux share in partthe same permeable path along said flux-conductor, an optical elementattached to said armature in the vicinity of said support axis andarranged to receive a beam to be directed by said optical element, and acircuit for controlling said drive means to produce a varying flux insaid flux-conductor to induce a varying current in said closed movableelectrical conductive loop to cause said armature and attached opticalelement to rotate back and forth about said support axis, in resonantscanning motion.
 2. The device of claim 1 wherein said optical elementcomprises a planar mirror mounted on the side of said loop opposite fromthe location of said permanent magnet, said mirror, armature and supportmeans therefor comprising a resonant assembly that is mass-balanced sothat acceleration produces substantially only translation of said planarmirror.
 3. An actuator comprisingan armature which includes a movable,closed electrically conductive loop, said armature being mounted torotate about a support axis, current inducing means associated with saidelectrically conductive loop comprisinga stationary flux-conductorextending through and in current-inducing relationship with said loop,and drive means adapted to induce a magnetic current-inducing flux alongsaid flux-conductor in accordance with desired movement of saidarmature, permanent magnet biasing means comprising a permanent magnetadapted to produce a magnetic field with which current in saidelectrically conductive loop interacts to cause movement of saidarmature,one pole of said magnet being permeably connected to oneportion of said flux-conductor at a region remote from said electricallyconductive loop, and the other pole of said magnet being permeablyconnected with said flux-conductor via a permeable gap in which asegment of said electrically conductive loop is arranged to move,wherebybias magnetic flux and current-inducing flux share in part the samepermeable path along said flux-conductor, and a scanning elementattached to said armature in the vicinity of said support axis andarranged for back and forth resonant motion in relation to a scanningaxis. a circuit for controlling said drive means to produce a varyingflux in said flux-conductor to induce a varying current in said closedmovable electrical conductive loop to cause said armature to rotate backand forth about said support axis, in resonant scanning motion.
 4. Thedevice of claim 1 or 3 further comprising an armature-supportingstructure comprising an elastic flexural support constructed andarranged to impose a restorative force tending to return said armaturefrom a deflected position to its neutral position, said armature andsaid armature-supporting structure having a resonant frequency ofrotational motion.
 5. The device of claim 4 wherein said elasticflexural support comprises a torsion bar assembly.
 6. The device ofclaim 5 wherein said torsion bar assembly comprises a pair of coaxial,axially spaced apart torsion bars with said armature held between therespective ends of said bars, said torsion bars defining said supportaxis.
 7. The device of claim 6 wherein an elongated supporting elementaligned with said axis is connected between said torsion bars, a planarmirror mounted on one side of said supporting element, and said loopextending, in perpendicular relationship to said mirror, from theoppositely directed side of said supporting element.
 8. The device ofclaim 4 wherein the motion of said armature and associated movingstructure is characterized by a (Q) (quality factor for a resonantsystem) of at least
 200. 9. The device of claim 1 or 3 wherein saidcircuit for controlling said drive means comprisesa sensor responsive torotation of said armature, and feedback circuitry responsive to saidsensor for providing control signals to said drive means, said sensorcomprising a pickoff coil wound around said flux-conductor for providinga signal indicative of the flux induced in said flux-conductor byrotation of said armature.
 10. The device of claim 9 wherein saidfeedback circuitry comprises an adjustable gain means for controllingthe amplitude of rotational excursion of said armature.
 11. The deviceof claim 1 or 3 wherein said drive means comprises a coil wound aroundsaid flux-conductor.
 12. The device of claim 1 or 3 wherein saidflux-conductor is of symmetrical form having a pair of legs spaced fromand in flux-conducting relationship with said permanent magnet, a coilwound about each of said legs, one of said coils connected to serve assaid drive means and the other of said coils arranged to serve as pickoff for sensing the velocity of said armature.
 13. The device of claim 1or 3 wherein said flux-conductor and said permanent magnet compriserespective opposed surfaces parallel to said support axis, defining saidgap, said conductive loop comprising a segment disposed within said gapin a direction parallel to said support axis and normal to flux linesproduced by said permanent magnet and passing through said gap betweensaid surfaces.
 14. The device of claim 13 wherein said opposed surfacesare arcuate, concentric surfaces of different radii centered on saidsupport axis, whereby the force on said loop caused by the interactionof said permanent magnetic field and said induced current in said loopis uniform regardless of the angular position of said segment of saidloop within said gap.
 15. The device of claim 1 or 3 constructed so thatsaid movement of said armature is produced by a torque resulting fromthe interaction of said magnetic field and the current in said loop,said torque being of the form

    T=KBI.sub.a

where B is the field strength of said permanent magnet, K is a constant,and I_(a) is the current induced in said loop, and wherein the motion ofsaid armature causes a damping torque of the form

    T.sub.D =FB(dθ/dt)

where F is a constant dependent on the configuration of said loop andsaid flux-conductor and θ is the angular position of said armature. 16.The device of claim 15 cnstructed so that damping both as a result ofsaid magnetic damping torque dependent on the field strength of saidpermanent magnet, and mechanical damping are so minimized as to providea (Q) (quality factor for a resonant system) of the order of 1,000 ormore.